Onii-chan's name is not specified since "Experimental Comic Kotone" started as a love simulation game. In games like "Tokimeki Memorial", main character's name is left blank since player usually put his/her name. So Onii-chan's name is and will be unknown forever.

Kotone-chan calls "Onii-chan", Konstantin calls him "My Brother", Laika calls him "You, Hey", and so on. I really have difficult time making other characters call him...